High aspect ratio tabular grains exhibit several pronounced photographic advantages. Thanks to their particular morphology greater amounts of spectral sensitizers can be adsorbed per mole of silver halide if compared with classical globular grains. As a consequence such spectrally sensitized tabular grains show an improved speed-granularity relationship and a wide separation between their blue speed and minus blue speed. Sharpness of photographic images can be improved using tabular grains thanks to their lower light scattering properties, again if compared with conventional globular emulsion grains. In colour negative materials e.g. the conventional sequence of the light-sensitive layers can be altered and the yellow filter layer can be omitted. In developed black-and-white images high covering power is obtained even at high hardening levels. Alternatively reduced silver halide coverages can be achieved if desired, which again results in improved sharpness. In duplitized radiographic materials the presence of tabular grains reduces the so-called cross-over which is the main factor for sharpness in such materials. Moreover coating amounts of silver can be reduced, further in favour of production cost and ecology.
An emulsion is generally understood to be a "tabular grain emulsion" when tabular grains account for at least 50 percent of the total grain projected area. A grain is generally considered to be a tabular grain when the ratio of its equivalent circular diameter to its thickness is at least 1.5. The equivalent circular diameter of a grain is the diameter of a circle having an area equal to the projected area of the grain.
Early patent disclosures on high aspect tabular grains, e.g. U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425; 4,425,426; 4,433,048 and Research Disclosure, Vol. 225, Jan. 1983, Item 22534, are concerned with high sensitive silver bromide or silver iodobromide {111} tabular grain emulsions. In a lot of photographic applications however high sensitivity is less important. In these cases the use of emulsions rich in chloride is advantageous thanks to their higher development and fixing rates favourable in rapid processing applications. Typical examples include graphic arts contact materials, duplicating materials, hard-copy materials, diffusion transfer reversal materials and black-and-white or colour print materials. However when combined, high sensitivity and rapid processing applicability are highly appreciated. So it remains interesting to combine the advantages of emulsions rich in chloride with the advantages of a tabular grain structure.
Silver halide tabular grains rich in chloride can have parallel faces in the {111} crystal plane or in the {100} crystal plane, thus providing a tabular {111} or a tabular {100} habit respectively.
In earlier disclosures most attention was paid to the preparation of tabular grains rich in chloride having a {111} crystal habit as in U.S. Pat. Nos. 4,400,463; 4,713,323; 4,804,621; 5,183,732; 5,185,239; 5,178,998; 5,178,997 and in EP-A 0 481 133.
The first publications on tabular grains bounded by {100} parallel major faces were related with silver iodobromide emulsions. Bogg in U.S. Pat. No. 4,063,951 and Mignot in U.S. Pat. No. 4,386,156 were the most important publications.
In EP-A 0 534 395 Brust et al. disclose the first {100} tabular emulsion grains rich in chloride and a process for preparing them wherein the tabular grain fraction showing {100} major faces is significant. Further improvements and variations on the teachings of the said tabular {100} emulsions rich in chloride have been described in U.S. Pat. Nos. 5,024,931; 5,264,337; 5,275,930; 5,292,632; 5,310,635; 5,314,798; 5,320,938; 5,356,764; 5,601,967; in WO-Applications 94/22051 and 94/22054 and in EP-A's 0 569 971; 0 584 815; 0 584 644; 0 602 878; 0 616 255; 0 617 317; 0 617 320; 0 617 321; 0 617 325; 0 618 492; 0 618 493; 0 653 659 and 0 653 669.
In conventional photographic materials for radiographic recording high-sensitive silver (iodo)bromide tabular emulsions are currently used. However with respect to recent trends to rapid processing applications it is desirable to use silver halide emulsions rich in chloride as the said emulsions show a faster developability as has e.g. been disclosed in EP-A 0 678 772.
One of the major problems arising in the preparation methods of {111} tabular grains rich in chloride is the problem of crystallographic stability, which after making use of a crystal habit modifier in the preparation step of the said grains requires the cumbersome step of replacing the said habit modifier by other compounds adsorbed at the large crystal surface as has e.g. been demonstrated in U.S. Pat. No. 5,221,602. Due to the steps of adsorbing, desorbing and replacing different adsorbing compounds the reproducibility and stability of the grains is questionable.
As has been shown e.g. in EP-A 0 653 669 during the preparation of {100} tabular grains rich in chloride the presence of such an adsorbed crystal habit modifier is not required as an excellent crystallographic stability is obtained. Moreover an improved reproducibility of sensitometric characteristics, if compared with equivalent {111} tabular silver halide emulsion crystals can be expected.
As it has always been important to get a percentage of tabular grains as high as possible within the whole emulsion crystal population, in favour of all properties offered by the said tabular grains, it is clear that every improvement in that direction is highly appreciated. An attempt to reach that object, particularly for high chloride {100} tabular grains comprising iodide ions, has been described in U.S. Pat. No. 5,413,904, wherein it has been proposed as an indispensable asset to delay the introduction of iodide ions in the reaction vessel until after grain nucleation has occurred.
As moreover tabular grains having higher aspect ratios and a reduced thickness are more favourable with respect to the amount of coated silver halide required in order to get the same covering power, speed and gradation within a shorter processing time if compared with thicker crystals having a lower aspect ratio, such thinner crystals having higher aspect ratios are highly preferred.
Moreover reduction of the presence besides the desired {100} tabular grains of grains having a habit deviating from the desired one as e.g. cubic grains or substantially cubic grains (having an aspect ratio of less than 1.5), needles (having a ratio of long edge length L to short edge length 1 of the cylinder of more than 10) and single twins (cubic {100} crystal having 1 single twin plane along &lt;111&gt;, &lt;311&gt; or &lt;411&gt; plane) is desired as well in favour of homogeneity of crystal habit.
The present invention thus further extends the teachings on tabular emulsions grains (or crystals) rich in silver chloride having a {100} crystal habit (having a ratio of long edge length L to short edge length 1 of the rectangle of not more than 10 and, more preferably not more than 5), more particularly teachings with respect to grains having an average aspect ratio of more than 5, an average equivalent grain or crystal diameter of at least 0.3 .mu.m and a thickness of less than 0.25 .mu.m.